Wednesday, September 9, 2015

California Heat Wave While Much Cooler Temps overspread the East

By: Steve Gregory , 8:45PM,GMT on September 9,2015






WEDNESDAY, 09-SEP-15 / 3:40 PM CDT
NEXT UPDATE ON FRIDAY (SEP 11TH)

RECORD HEAT IN CALIFORNIA AS DYNAMIC UPPER LEVEL PATTERN TAKES HOLD

With an amplifying TROF over the center of the nation, the well above normal Temps in the eastern US will be coming to abrupt end during the next few days as a seasonally strong cold front moves from the Ohio / Mississippi Valley region to off the east coast by the weekend. Locally heavy rains are likely to accompany the frontal passage, with widespread 1”+ totals likely.

Meanwhile, an offshore flow in California brought a ‘typical’ early Fall Heat Wave to the coastal cities in California, with record highs set yesterday in both SFO and downtown LAX. The relatively strong offshore flow will again send Temps up to the mid 90’s in the SFO Bay area and close to 100˚ at many locations in the LAX basin today and Thursday. The heat will start to subside by FRI, but ‘normal’ Temps are not likely to return until the start of next week as the marine layer advances into the coastal areas.

While the eastern US TROF will continue to deepen through the weekend, it will gradually advance towards the east as the overall hemispheric flow pattern is, and will likely remain, quite progressive. This will cause the eastern TROF to de-amplify as it moves out over the Atlantic and will be followed by a more zonal-like flow next week. Further upstream, the next short-wave now moving towards Alaska will then move southeastward into the PAC NW and eventually the central US in about 8-10 days – amplifying into another high amplitude TROF in the central/eastern US during Week 2. At the same time, remnant moisture from what is now Hurricane LINDA southwest of Baja, CA will likely move towards Texas and could lead to locally heavy rainfall late next week – especially if it gets absorbed by - and interacts with - the next amplifying TROF.

While there continue to be significant day-to-day differences in model forecasts – overall – the model projections are far more consistent now than they’ve been in quite some time. With this in mind, it’s worth noting that the global models (both the GFS and ECMWF) and their ensembles remain in generally very good agreement that a western US TROF is likely to develop towards the end of the month. If this occurs, a relatively warm end to the month is likely across the eastern US.

TROPICAL ATLANTIC CONTINUES TO ACT LIKE IT’S JUNE

Tropical Storm GRACE weakened to a Depression yesterday – and has already dissipated into a weak low level TROF in the central Atlantic.

With the exception of a very small and inconsequential Depression that formed to the east of Bermuda yesterday, the tropical Atlantic remains unusually (but not unexpectedly) ‘quiet’. Between higher than normal wind shears and unusually stable air across much of the tropical Atlantic – cyclone activity is likely to remain subdued through the rest of the season.

ALASKA WEATHER

As expected, upper level heights across the state have lowered, with relatively strong TROFs and upper level Lows expected to encompass the entire state for the next 2 or more weeks. Strong short waves moving eastward from Siberia and near the Kamchatka Peninsula every few day, will intensify as they move across southern Alaska – with most of the ‘energy’ associated with each system moving from the Aleutians eastward across the northern GOA before diving southeastward towards the PAC NW. While this pattern will prevent any significant warm-ups – the rapidly moving systems will also prevent the formation (or advection) of unusually cold air from developing across the state anytime soon. However, widespread Precip will dominate the weather for the next 2 weeks – with relatively heavy Precip expected across far southern locations – especially south of the coastal mountain ranges - with the next especially strong storm expected during the middle of next week.



CLICK IMAGE To Open FULL Size image in a new window
Fig 1: Overview of Tropical Atlantic BasinThe weak Tropical Waves in and approaching the CARIB have moderate moisture associated with them – but high shears and a stable environment nixes any chances for development. What WAS Tropical Storm GRACE weakened to a Depression yesterday and essentially dissipated today as stable air and wind shear destroyed the system. A large Tropical Wave near the Cape Verde Islands is westbound, but conditions are unfavorable for development there as well. Finally, the small disturbance that spun up from an old/decaying frontal boundary into very weak Tropical Depression #8 is about 250NM east of Bermuda and appears to be quasi-stationary. The system is ‘surrounded’ by dry air – with moderate shears of 10-15 Kts. However, during the next couple days, the system will likely degenerate into a wave or at least non-tropical Low as it gets absorbed by an approaching mid-latitude system. Unless there's a sudden surprise – I will not be covering this system again.



Fig 2: The TPW (Total Precipitable Water) Loop (Top) and latest image frame (bottom) with ‘V’ wind vector component analysis) highlights areas with deep layered moisture. ‘Bluish’ colors represent dryer air while the darker, ‘orange’ tones highlight areas of deep moisture in the column of air above the surface. The 72 hour looped imagery highlights the rotation associated with GRACE as it moved across the Atlantic (and is now defunct!) and the formation of TD#8 that formed along a frontal boundary to the east of Bermuda. The TW that moved off the African coast yesterday has lost most of its rotation and convection, while the next upstream system that looked fairly impressive 2 days ago – has also weakened considerably – with only diurnal based convection appearing. The lower image analysis includes the ‘V’ component of the Wind Vectors - still shows some rotation associated with the TW approaching the African coast, but has lost much of its convection. While not shown - tropical waves across Africa remain unusually far south for this time of year, with most of them moving along 10N latitude. (Normally, they move closer to 15N latitude during the seasonal 'peak' of hurricane season.)


Fig 3: IR image and satellite derived wind analysis from CIMSS over the EPAC. Hurricane LINDA west of Baja will continue moving NW and will eventually dissipate. The lower level circulation will 'decouple' from the upper level circulation field as the system weakens - with the moisture expected to advect northeastward across northern Mexico towards NM and TX during the weekend. The tropical moisture may then become entrained with an approaching mid-latitude TROF later next week – leading to a locally heavy rainfall event.


Fig 4: VIS image of LINDA and western US region. The ‘true-color’ image from the MODIS satellite platform shows the fog/stratus that typically hugs the California coast has pushed far to the west due to the large scale offshore flow (easterly wind flow). The down sloping winds have helped send Temps to triple digit levels right up to the coast. The marine layer should return to the coastal areas by the start of next week.


Fig 5: 250MB Wind/Jet Stream Loop The above loop (still in the ‘development’ stage) shows the expected evolution of the upper air jet stream pattern during Week 1.


Fig 6: Temperature ANOMALY forecast for Week 1 is based STRICTLY on the MOS data forecasts from the todays 12Z operational GFS model run with only minor adjustments towards the raw model data points for Days 6 and 7. Unseasonably warm Temps are coming to an end in the eastern US as a deep TROF develops over the central/eastern US. Confidence in the forecast anomaly Pattern and Magnitude is near average for this time of year, with readings of ‘3’ for both the anomaly Pattern and Magnitude on a Scale from 1 to 5.


Fig 7: Temperature ANOMALY forecast for Week 1 is based STRICTLY on the MOS data forecasts from the todays 12Z operational GFS model run with only minor adjustments towards the raw model data points for Days 6 and 7. A fairly typical SEP Temp pattern across the state – with Temps averaging near to slightly above normal over the far south central region (The Anchorage region). Little change to the anomaly pattern is expected during Week 2. (Week 2 Temp anomaly charts for Alaska will likely begin on OCT 2.)


Fig 8: Running Precipitation totals for US during the next 10-days (and only for totals > 0.75”) based STRICTLY on the GFS Moderately heavy Precip is expected over the eastern third of the nation during the next 10-days as 2, cold fronts move across the region during the next 10 days. Storm systems near Alaska will produce locally heavy Precip across the southern portion of the state – and widespread, but relatively light Precip – in the north. Very Heavy Precip shown over Mexico on into SE Texas is due to remnant moisture from LINDA.


Fig 9: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (80%) integrated with the 12Z GFS Ensemble (10%) and the 00Z EURO model Ensemble (10%) using the projected pattern along with explicit surface and 850mb (~5,000 FT) Temp forecasts. Some Temps are adjusted for known / expected anomalous thermal patterns & projected storm system passages. Much cooler Temps will prevail in the Midwest during the beginning of Week 2 – but Temps should cool down in the far west and warm up in the central US by the end of Week 2. Overall Confidence in the forecast anomaly Pattern is near average now as run-to-run model consistency has improved considerably over the past few days. But Confidence in anomaly Magnitudes is still below average for Week 2, with a reading of ‘2’ on a Scale of 1 to 5.

✭ The Next WX Update is Scheduled for FRIDAY, SEP 11TH ✭

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Steve

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